In wireless communication systems, the task of deciding whether to admit or reject a request for a radio bearer setup is handled by a function denoted Admission Control. The admission control decides whether to accept or reject a request for a radio bearer setup based on the present system load, e.g. in form of usage of certain monitored system resources (MSRs) as well as the type of bearer the request relates to.
An example situation is illustrated in FIG. 1, which shows a base station BS 101 in a cell 103 in a wireless communication system. A number of mobile terminals 102:1-102:5 are located in the cell 103. The mobile terminals 102:2-102:5 have ongoing services, for which the BS 101 have set up a number of radio bearers. The services may be of different types. If the user of e.g. the mobile terminal 102:1 wants to start a service, such as a VoIP call, a request is sent from the mobile terminal 102:1 to the base station 101 serving the mobile terminal, requesting resources for the VoIP call. This may be referred to as that the mobile terminal 102:1 sends a request for a bearer setup. The base station receives the request, and evaluates the current load in the cell 103. If it is determined that there are enough resources available for allowing setup of a VoIP call in the cell, the request is accepted, and a radio bearer is set up for the VoIP call. On the other hand, if it is determined that the current load is too high for allowing a VoIP call, the request for resources is rejected.
Such radio bearers are sometimes denoted Radio Access Bearers (RABs), or, in EPS systems, E-RABs. A radio bearer may be regarded as a virtual connection between two endpoints, which provides a transport service. The transport service, and thus the bearer, may be associated with specific QoS attributes. For example, data packets related to a conversational call should preferably not be delayed more than a certain number of ms, for obvious reasons. Thus, a radio bearer set up for a conversational call may be assigned a QoS attribute related to e.g. a guaranteed bitrate and a maximum packet delay.
Different services have different demands on e.g. bit rate and delay. A distinction is made between services with QoS requirements, supported by QoS bearers, and services without any QoS requirements, where services without QoS requirements are referred to as “best effort services”. Services with QoS requirements are services such as VoIP that need at least a certain bit rate to function at all. For a best effort service, such as normal FTP traffic, the end user is considered satisfied as long as the bit rate is not zero.
Given that reasonable spectral efficiency is achieved, the scheduler has the task of dividing the available resources between different users and services with the goal of fulfilling the QoS requirements of bearers in the system.
QoS guidelines [1] state that when the QoS demands of a user are in danger of being violated resources shall be assigned according to priority levels. For best-effort services, these guide-lines can be interpreted such that best-effort are not prioritized at all, since it lacks a strict QoS requirement, or alternatively has a low priority level compared to QoS bearers.
The relation between Admission Control and QoS is that Admission Control can limit the amount of traffic in order to protect the service of existing QoS bearers, with strict QoS requirements, by introducing a threshold for the MSRs, such that QoS contracts of admitted users shall be broken only infrequently. The reasoning behind such thresholds is that if usage with respect to a certain MSR approaches its max capacity, the probability of not finding resources to fulfill QoS contracts increases. The difference between the actual usage of an MSR and the max capacity of said MSR represents a headroom that can absorb fluctuations like incoming mobility, intra-cell mobility and varying radio conditions. Therefore, a threshold for admission control will create this headroom and it can be configured in accordance to what fluctuations are expected for the deployment. A side-effect is that the threshold can also be used to set aside resources for best-effort traffic.
An illustration on how Admission Control compares the load to the threshold is given in FIG. 2. The main purpose of admission control is to disallow excessive load on resources maintained by scheduling, such that users already admitted in the system may be kept satisfied. High load is identified by monitoring the usage of certain monitored system resources. FIG. 2 shows how blocking may be triggered in admission control, i.e. how requests for GBR (Guaranteed Bit Rate) bearer setup are rejected during the period 202, when the usage of a certain system resource exceeds a threshold.
In LTE systems, shared channels are used, as opposed to dedicated channels used in many other communication standards. When applying shared channels, a number of users and/or services compete for the same resources. These resources include e.g. physical resource blocks (PRBs) on Physical Downlink Shared Channel/Physical Uplink Shared Channel (PDSCH/PUSCH) and control channel elements (CCEs) on the Physical Downlink Control Channel (PDCCH). There is a dynamic aspect of these resources in the sense that the consumption, e.g. by one E-RAB, of these resources can vary from one TTI to another.
The inventors have identified problems associated with Admission Control in systems applying shared channels, which problems will be further described below. It is identified as a difficult but important task for Admission Control to make appropriate admission decisions in shared channel systems, such as LTE.